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annoying feed back need help getting rid of it ? Answered

ok so i have an old guitar amp with 2 car speakers and a drill battery and a car inverter. somewhere in this line im getting feedback   like buzzing and it sucks   i made this thing in hopes of having a really loud back pack sound system and it worked very well but the feed back needs to go i dont know what causes it but i know it has something to do with the way im powering it because when i use a wall plug instead of the inverter it stops the buzzing but how do i fix it for good


Ferrite beads and audio ground loop eliminators ("ground buster" transformers) only mask the cause of the interference. Again, the cause is in the kludged power supply of the drill battery and inverter.

1. The inverter is most likely an inexpensive, low power, square wave power inverter. This means that there is probably a good amount of harmonics radiating out of the inverter itself.

2. The drill battery, no matter what the voltage rating, will be underpowered for the inverter, leading to even more erratic operation of the already suspect inverter.

Now, since the old amp uses a line power cord (not a wall wart), and does not have the interference/hum/noise when using the line power cord, all of the "problems" must be attributed to the inverter and battery. Since the battery only produces DC, then the real problem has to be the inverter. As I stated earlier, why go through the inefficiency of inverting up to AC line voltage and then back down to DC again when you can just tap the correct DC voltage into the circuit in the first place. The battery pack will last a lot longer, as some of those cheap inverters can pull several amps when sitting idle. If you forget to disconnect/turn off the battery, it'll be dead in no time.

I say, why mask the problem when you can eliminate it completely and gain efficiency in the process.


I dont know much about A/C electronics, but if it was D/C you would use a diode. I dont think it will work.

I have a lot of success getting rid of HUM by using a "ground-buster" transformer. All it is .... is a tiny little transformer for audio. Both sides of the transformer are the same impedance... 1000 ohms i think. Any audio at the primary of the transformer also appears at the secondary of the transformer through magnetic transfer. No electrical connection is made from one side of the transformer to the other side. This eliminates HUM in a lot of cases. Radio shack sells one that has RCA stereo wires on it. ... or... You can build one for free if you have one of those old dialup modems laying around. Just unsolder and remove the tiny audio transformer found in most modems. connect your RCA wires... and you are done! Goodbye hum.

I doubt its a powersupply issue, in the sense its underpowered, I suspect its pickup on the front end. Try putting some ferrite beads on the input leads as close to the actual amp as you can manage.

As Re and Ork have said, it is the inverter setup that is causing the hum/buzz. Since you have no noise when using the line plug that came with the amp (not a wall wart), you'll need to dig into the amp and find the power transformer and rectifier section (I assume this old amp is solid state, not tube) and measure the DC voltage there. Then you can make a battery with enough voltage to power it, connected at that same point.

It may also be that the drill battery does not have enough power to run the inverter and the amp at the same time. Some inverters are horribly inefficient. Try hooking it up to a car battery and see if the hum goes away. If it does, a larger battery should work, but the best way is still directly connecting a proper sized battery to the DC path in the amp's power supply.

Also, remember that a guitar amp is a high gain amplifier, and if the inputs are not shielded enough, you may get feedback or other noise. Just a heads up.


Find out what voltage the wall wart is and power it directly from the battery that way. Powering it thru the inverter is what's causing the buzz. The inverter isn't giving you clean a/c and then that goes into the wall wart and then into the amp.

If you power it directly from a battery there will be NO BUZZ.

+1. It isn't feedback, it's hum. I'd bet you're using a square-wave inverter, or at best one with a two-step waveform. That produces strong harmonics which your wallwart isn't designed to filter out. You either need a true sine-wave inverter (costs more), or heavy filtering, or -- as Re-design suggests -- you need to discard the whole AC part of the power chain and go direct through DC, or perhaps through a DC-to-DC converter if you need to step the voltage up. That last is definitely the best solution, since among other things it will use the power from the battery more efficiently.